Abstract

Gibberellins (GAs) are plant hormones that regulate most plant life cycle aspects, including flowering and fruit development. Here, we demonstrate the implication of GAs in ovule development. DELLA proteins, negative GA response regulators, act as positive factors for ovule integument development in a mechanism that involves transcription factor ABERRANT TESTA SHAPE (ATS). The seeds of the della global mutant, a complete loss-of-function of DELLA, and the ats-1 mutant are remarkably similar, with a round shape, a disorganized testa, and viviparism. These defects are the result of an alteration in integuments that fail to fully develop and are shorter than in wild-type plants. ats-1 also shows some GA-related phenotypes, for example, higher germination rates and early flowering. In fact, ats-1 has elevated GA levels due to the activation of GA biosynthesis genes, which indicates that ATS inhibits GA biosynthesis. Moreover, DELLAs and ATS proteins interact, which suggests the formation of a transcriptional complex that regulates the expression of genes involved in integument growth. Therefore, the repression of GA biosynthesis by ATS would result in the stabilization of DELLAs to ensure correct ATS-DELLA complex formation. The requirement of both activities to coordinate proper ovule development strongly argues that the ATS-DELLA complex acts as a key molecular factor. This work provides the first evidence for a role of GAs in ovule and seed development.

Gibberellins (GAs) are plant tetracyclic diterpenoids that play a major role in diverse key developmental processes throughout the plant life cycle, including seed germination, stem and root elongation, flowering, and fruit development (Sun, 2010; Gupta and Chakrabarty, 2013). The master regulator in GA signaling is the DELLA protein, a subfamily of the plant-specific GRAS family of transcriptional regulators (Sun, 2010). Bioactive GAs are perceived by receptor GID1s to allow GA-GID1-DELLA complex formation, which results in structural changes in the DELLA protein that trigger recognition and binding with F-box proteins, polyubiquitination, and subsequent degradation by the 26S proteasome (Sun, 2011). Therefore, while DELLA proteins act as plant growth repressors, GAs promote growth and development by the rapid degradation of DELLA proteins. Accordingly, lack of DELLA activity in different mutants of Arabidopsis (Arabidopsis thaliana; Cheng et al., 2004; Feng et al., 2008), tomato (Solanum lycopersicum; Jasinski et al., 2008; Bassel et al., 2008), or rice (Oryza sativa; Ikeda et al., 2001) results in a constitutive GA response. On the contrary, the mutant alleles of the DELLAs that lack the amino domain DELLA, like gai-1 (Peng et al., 1997) or pRGA:GFP-rgaΔ17 (Dill et al., 2001) of Arabidopsis and Slr1-d in rice (Asano et al., 2009), encode proteins that cannot be degraded, which results in constitutive DELLA activity and GA response blockage. Unlike most analyzed plant species that encode a single DELLA protein, the Arabidopsis genome encodes five DELLAs: GA-INSENSITIVE (GAI), REPRESSOR OF ga1-3 (RGA), RGA-LIKE1 (RGL1), RGL2, and RGL3. Each one performs distinct, but also overlapping, functions in repressing GA responses (Sun, 2011).

DELLA proteins function as central nodes that integrate hormonal and environmental cues to modulate transcriptional patterns, which finally regulate growth and development (Davière and Achard, 2016). DELLAs lack a canonical DNA binding domain and thus mediate the transcriptional regulation of the target genes involved in the GA response throughout the direct physical interaction with transcription factors (TFs) and other regulatory proteins (Vera-Sirera et al., 2015; Davière and Achard, 2016). DELLA-TF binding can be divided into two major mechanisms. DELLAs can bind to either TFs or other transcriptional regulators to block their function, or bind to TF linked to the promoter of target the genes, modulating their transcriptional activity (Davière and Achard, 2016). The list of potential DELLA interactors has rapidly increased and provided direct evidence for not only the mechanism of crosstalk between GAs and other hormones, but also for environmental clues to modulate adequate plant growth and response to environmental conditions (Marín-de la Rosa et al., 2014, 2015; Davière and Achard, 2016).

Despite all these data, very little is known about the role of GAs during early pistil ontogeny, except for valve margin differentiation (Arnaud et al., 2010). ALCATRAZ (ALC) is a bHLH protein involved in the formation of the dehiscence zone in the valve margin, required for fruit shattering and seed dispersal. DELLAs block ALC activity in the specification of the separation cell layer in the dehiscence zone by means of direct DELLA-ALC protein binding.

No previous evidence has implicated the GAs in ovule initiation and formation, a key developmental process that occurs during early pistil ontogeny. Ovules emerge from the placental tissue as finger-like primordia at stage 8 of flower development (Smyth et al., 1990; Modrusan et al., 1994). Later, three different regions can be distinguished along the proximal-distal axis: the funiculus, which attaches the ovule to the placenta, the chalaza, and the nucellus, which encloses the megaspore mother cell. Integuments develop asymmetrically from the chalaza surrounding the nucellus. The outer integument totally overgrows the inner integument in the mature ovule. Upon fertilization, cells in both the outer and the inner integument undergo a transformation process, which gives rise to the testa or seed coat (Western et al., 2001; Haughn and Chaudhury, 2005). Therefore, the structure and morphology of the mature testa depend on the correct initiation and growth of integuments. Consequently, regulation of integument development can have vast effects on the final testa structure and composition.

Numerous genes involved in integument patterning and growth have been identified (Sieber et al., 2004; Battaglia et al., 2009; Kelley and Gasser, 2009). One such gene is ABERRANT TESTA SHAPE (ATS, At5g42630; McAbee et al., 2006), which encodes a KANADI (KAN) TF, previously named KAN4. ATS provides boundary maintenance and promotes the laminar growth of the inner ovule integument. Loss of the ATS function in the ats-1 mutant leads to the fusion of the inner and outer integuments that grow as a unit to produce a single fused structure. As a result of this fusion, ats-1 seeds are abnormally rounded and variable in size (Leon-Kloosterziel et al., 1994; McAbee et al., 2006).

Here, we describe the implication of GAs as negative factors in integument growth during ovule ontogeny and how DELLAs are required for correct integuments formation. DELLA activity would be mediated by its direct interaction with the KANADI TF ATS/KAN4. Our data suggest that the ATS-DELLA complex is a key molecular factor for regulating the transcription of the genes involved in ovule integument growth. The positive role of DELLAs and the detrimental effect of constitutive GA signaling on ovule development contrast with the traditional view of the DELLAs as negative growth regulators. This is the first evidence to our knowledge for the implication of DELLAs in the development of ovules. We propose a molecular model of the function of DELLAs and ATS during integument grow.

RESULTS

DELLAs Participate in Seed Formation

Constitutive GA signaling in Arabidopsis multiple DELLA loss-of-function mutants gives rise to fruits with fewer seeds and shorter in length than those in Ler (wild-type) plants (Cao et al., 2005; Dorcey et al., 2009). To gain a better picture of the role of GAs in seed development, we thoroughly analyzed the number and morphology of the seeds in the global mutant (the quintuple gaiT6 rgaT2 rgl1-1 rgl2-1 rgl3-1 mutant). All the assays were carried out in fruits from emasculated flowers pollinated with wild-type pollen to ensure that only maternal effects were tested. The seed number of the global mutant decreased by 60%, while fruit length shortened only by 30%, which resulted in lower seed density per fruit (Fig. 1, A and B). In many species, including Arabidopsis, fruit size correlates with seed number (Cox and Swain, 2006), which suggests that the facultative parthenocarpy of the global mutant, due to constitutive GA signaling, may account for the further elongation of these fruits and lower seed density. While mature fruits of wild-type plants had seed-filled siliques, the fruits of the global mutant had many aborted seeds (Fig. 1C). More importantly, the global mutant mature seeds were irregularly round in shape and smaller than the wild-type seeds (Fig. 1, C–E). The della quadruple mutant of GAI, RGA, RGL1, and RGL2 showed a similar phenotype (shorter fruits, fewer and rounded seeds, and parthenocarpy), which indicates that the function of these four DELLA proteins was required for proper seed development and that RGL3 was not involved (Supplemental Fig. S1; Dorcey et al., 2009). In contrast, any of the four triple della mutants of GAI, RGA, RGL1, and RGL2 showed parthenocarpy or defects in seed morphology, which would suggest that both processes share a common genetic basis (Supplemental Fig. S1).

Constitutive GA signaling in the della global mutant causes seed defects and reduced fertility, similarly to ats-1. A, Silique length, seed number, and length/width ratio in fruits of Ler and the global and ats-1 mutants at 12 dpa. Data are the mean and sem of three independent experiments, each one from at least 50 fruits. The significant differences (Student’s t test analysis) between Ler and mutants are marked with asterisks (*P value < 0.01). B and C, Images of whole (B) and open (C) 8-dpa fruits of Ler, and the global and ats-1 mutants. D and E, Images of mature seeds of Ler, and the global and ats-1 mutants, taken by scanning electron microscopy (D) or stereomicroscope (E). The white arrow marks the viviparous seeds of the ats-1 and global mutants. All the images in each category have the same magnification. Scale bars = 2 mm (B), 500 µm (C and E), and 100 µm (D).

The testa of the global mutant seeds was defective (Fig. 1D). The epidermal cells in the testa are characterized by polygonal structures with a central elevation or columella. In contrast, the epidermal cells of global mutant seeds clearly lacked this structure and were less regular in shape. These modifications may diminish the mechanical restriction to embryo growth and collaborate with GA constitutive signaling in the viviparism occurrence (germination inside the silique) in global mutant seeds (Fig. 1E). Alteration in the testa and seed shape of the global mutant did not interfere with seed viability, which indicates that embryo development is not altered (see below for seed germination assays). All defects in the global mutant seeds are exclusively attributable to maternal tissue, as all the assays were carried out in emasculated flowers that were hand-pollinated with wild-type pollen.

DELLA activity during seed formation was also tested using the gain-of-function gai-1 mutant. The gai-1 seeds showed apparently unaltered seed morphology, but were 25% larger than those of the wild-type due to an increase along the major axis (Fig. 2, A–C), in contrast to the global seeds that were 25% shorter in length than wild-type seeds (Fig. 2, B and C). No significant defects were detected in the width of seeds from the global or gai-1 mutants. It has been reported that length and width of Arabidopsis wild-type seeds follow the Golden ratio (Cervantes et al., 2010), also known as the divine proportion, a mathematical constant (Φ = 1.618) frequently found in many biological-based geometries. The length/width ratio of the wild-type seeds was 1.60, which came very close to the Golden ratio. In contrast, the ratio in the gai-1 and global mutants was 1.75 and 1.19, respectively, which reflects the enlarged and shorter seed shape caused by the gain and loss-of-function of the DELLA functions. These results indicate that DELLA activity is required for proper seed growth and development, mainly on the proximal/distal axis.

DELLAs and ATS regulate seed size and shape. A, Scanning electron microscopy images of mature seeds of Ler and the gai-1 mutant. Length and width (major and minor axis, respectively) are market with white arrows in Ler seed. B, Size distribution of mature seeds of Ler and the gai-1, global, and ats-1 mutants, represented as length (major axis) and width (minor axis). Golden ratio is represented by a line at the 1,618 ratio. C, Seed length and width of Ler and the gai-1, global, and ats-1 mutants. Data are the mean ± sem of three biological replicas, each one from 25 to 30 seeds. Significant differences (Student’s t test analysis) between Ler and mutants are marked with asterisks (*P value < 0.01). The ratio between seed length and width is indicated.

The della global Mutant Has Defects in Integument Development

The testa is differentiated from ovule integuments (Beeckman et al., 2000). Therefore, the majority of mutants with structural defects in the testa are affected in integument development. Figure 3 shows ovule and seed development in the global mutant and wild type. At early ovule development, no clear distinction was noticed (Fig. 3A), but altered growth of integuments was observed at stage 3-II in the global mutant (Schneitz et al., 1995). Integuments were shorter, with no clear distinction among their different cell layers. As a consequence, at anthesis, only two cell layers in each integument were formed, and the outer integument did not cover the nucellus, displaying an altered shape (Fig. 3C). In contrast to the global mutant, a wild-type ovule at anthesis is formed by two layers of cells in the outer integument and three layers of the inner integument; the outer integument has overgrown the shorter inner integument around the embryo sac. Developing seeds of the global mutant clearly showed unusual testa development, with three to four cell layers, unlike the five well-defined cell layers of a wild-type seed, with full or partial endothelium layer, depending on the severity of phenotype (Fig. 3, D and E). Therefore, the altered ovule development of the global mutant is the most possible cause of the abnormal seed morphology.

DELLAs and ATS regulate ovule integument development. A and B, Images of Ler, global, and ats-1 ovules at stage 2-III (A) and 3-II (B; Schneitz et al., 1995). C, Images of ovules at anthesis. The shape of the embryo sac (es) is marked by blue lines. D and E, Transversal section of the fertilized ovules at 3 dpa (D) or 7 dpa (E). The outer and inner integument layers of wild type in A and D are indicated with asterisks and dots, respectively. Scale bars = 50 µm (A–C) and 100 µm (D and E).

The Ovule and Seed Phenotypes of the global Mutant Resemble Those of the ats-1 Mutant

Similar alterations in mature ovules to those in the global mutant were observed in the null aberrant testa shape-1 (ats-1) mutant (Leon-Kloosterziel et al., 1994). ATS/KAN4 (At5g42630) encodes one of the four KANADI gene family members (Hawker and Bowman, 2004), a subset of the GARP (GOLDEN2, ARR-B Class, Par1 proteins) family of putative TF genes that play roles in leaf polarity and expansion and also in ovule development (Eshed et al., 2001; McAbee et al., 2006).

To facilitate comparisons, the data from ats-1 are shown along with data from the global mutant in Figures 1, 2, and 3. The siliques of ats-1 are shorter and contain fewer seeds that wild type, but unlike the global mutant, ats-1 showed a proportional reduction of seed number and silique length, resulting in a ratio similar to wild type (Fig. 1, A and B), as ats-1 does not display parthenocarpy (Vivian-Smith et al., 2001). The ats-1 siliques also contained aborted seeds, with defects in the testa and viviparism (Fig. 1, C–E), and were smaller, globular in shape, and had a length/width ratio of 1.12, (Fig. 2, B and C). Moreover, the ovules of ats-1 at anthesis resembled to those of the global mutant, with short integuments that did not cover the embryo sac at the micropyle and fewer integument layers (Fig. 3C). Unlike global mutant, ats-1 outer and inner integuments were fused, with no separation space between them from early developmental stages (Fig. 3A; Leon-Kloosterziel et al., 1994; McAbee et al., 2006). In contrast, at stage 3-II, integuments of ats-1 arrested growth and did not extend beyond the embryo sac, similarly to the global mutant (Fig. 3B). Defects in testa during seed development of both global and ats-1 were also very similar (Fig. 3, D and E). Although both global and ats-1 mutants showed different ovule integument development in early stages, we pursued the study of ATS role in the GA regulation of ovule growth based on two key observations: ATS was a potential binding protein to the DELLAs (Marín-de la Rosa et al., 2015) and ats-1 displayed typical GA-related phenotypes.

ats-1 Displays GA Signaling Phenotypes

We have shown that the ovule and seed defects of ats-1 closely resembled those of the global mutant. Interestingly, ats-1 also showed GA-related phenotypes (Fig. 4). The ats-1 plants had similar alterations in germination and flowering to those observed in plants with a constitutive GA signaling response. GAs play a central role in germination by promoting testa breakage and facilitating radicle protrusion. Loss of DELLA activity in the global mutant plants led to both light- and GA-independent seed germination, with RGL2 being the predominant repressor of seed germination (Lee et al., 2002; Cao et al., 2005). The ats-1 mutant seeds showed higher germination rate compared to those of the wild type but lower than those of the global and rgl2-1 mutants (Fig. 4A). However, the ats-1 seeds germinated at a similar rate to the null rgl2-1 mutant in the presence of the GA inhibitor paclobutrazol (PCB). Lack of ATS function overcame the seed germination inhibition produced by the stabilized DELLA proteins by PCB, which indicates that ATS plays a role as a repressor of germination. Finally, the alterations in ovule and seed morphology observed in the ats-1 and global mutants did not affect embryo viability as the germination rates were high, especially in PCB.

ats-1 shows higher seed germination and early flowering. A, Germination rate of Ler and the ats-1, global, and rgl2-1 mutants. Germination was scored at 3 d in the absence (−PCB) or presence of 1 µm of PCB (+PCB). B, Flowering time of Ler and the ats-1 and global mutants. Flowering was scored as the number of leaves in plants grown in long (LD) and short (SD) days. Significant differences (Student’s t test analysis) between Ler and mutants are indicated by an asterisk (*P value < 0.01). Data are the mean ± sem of three biological replicas, each one from 80 to 100 seeds in A and 30 to 40 plants in B.

GAs regulate flowering time and stem elongation, especially under short days (Galvão et al., 2012; Porri et al., 2012). Coinciding with a possible GA-related phenotype caused by the loss-of-function of ATS, the flowering time of the ats-1 plants advanced particularly in short days (Fig. 4B). The ats-1 mutant plants also flowered slightly earlier than the wild type under long days. Early flowering of ats-1 was very similar to that of the global mutant (Fig. 4B). Taken together, our data suggest that ATS and DELLA may participate in germination and flowering and also in ovule/seed formation, through a common molecular mechanism. These analyses uncovered new functions of ATS beyond the ovule and seed development reported previously. Interestingly, there is a discrepancy between the known ATS expression pattern and the spatial distribution of phenotypes, which would be further explained by possible indirect effects, non-cell autonomy of ATS function, or the lack of a comprehensive determination of the ATS expression.

ATS Inhibits GA Biosynthesis

We hypothesized that an increase in GA content in ats-1 could be the reason for the GA-related phenotypes described above. Direct quantification of the GA forms that belonged to the non-13-hydroxylated pathway, which contributes mainly to the biosynthesis of the bioactive GA4, supported a role of ATS in inhibiting GA biosynthesis (Fig. 5A). Levels of GA4 increased 3-fold in ats-1 compared with the wild type, while levels of intermediates GA12 and GA24, and the inactivated form GA51, were also significantly increased. Furthermore, expression of GA biosynthesis genes GA20ox2, GA3ox1, and GA3ox2, but not GA20ox1, was up-regulated in ats-1 (Supplemental Fig. S2). Next, we studied the expression pattern of GA3ox1, which catalyzes the last step in the biosynthesis of bioactive GA1 and GA4 (Talon et al., 1990). Expression was significantly increased in developing ovules of the ats-1 mutant (Fig. 5B). The increased GA3ox1 expression in ats-1 likely led to increase bioactive GAs, which finally might promote the alteration in ovule morphology. To analyze whether the increased GA content affected DELLA protein stability in ovules, we tested the RGA levels using the pRGA:RGA-GFP reporter construct (Silverstone et al., 2001). While RGA was located in the chalaza and integuments of developing ovules, levels were decreased in the ats-1 mutant (Fig. 5C). Furthermore, RGA-GFP levels were also decreased in both secondary roots, where ATS is also expressed (Hawker and Bowman, 2004), and the primary root (Supplemental Fig. S3). In conclusion, our data indicated a role of ATS in repressing GA synthesis; low GA levels would stabilize DELLAs to properly coordinate growth and development. The increased GA biosynthesis in ats-1 would result in DELLA degradation and the deregulation of GA-controlled processes, including germination, flowering time, or ovule development.

DELLAs Interact with ATS

DELLAs function as transcriptional regulators but do not encode any known DNA-binding domain. This role is exerted through their interaction with TFs. KAN1, a related KANADI gene family member, has been recently described as a putative DELLA interacting protein (Marín-de la Rosa et al., 2015). Therefore, a plausible hypothesis would be that ATS/KAN4 could also interact with the DELLA proteins. To address this hypothesis, we performed a yeast two-hybrid assay using truncated versions of GAI and RGA, which prevent the auto-activation of reporter genes (de Lucas et al., 2008) and ATS. The ATS locus encodes at least two splice variants: ATS.1, the longest with six exons, and ATS.2, which lacks the last two exons (Fig. 6A; Gao et al., 2010). Interaction assays demonstrated that only ATS.1 was able to interact with GAI and RGA (Fig. 6B). No interaction was observed for the truncated version ATS.2, which indicated that the last 67 amino acids at the carboxyl end of ATS.1 are an absolute requirement for DELLA-ATS binding and either act as the binding domain or stabilize the DELLA-ATS interaction. This domain does not encode for any known canonical protein motif. On the other hand, we also confirmed that GAI and RGA could also bind to KAN1 (Supplemental Fig. S4). We examined the subcellular localization of the GAI-ATS and RGA-ATS interactions by bimolecular fluorescence complementation (BiFC) and found that the reconstructed YFP protein was observed only in the nuclei of epidermal cells that coexpressed YFP-ATS.1 and YFP-GAI or YFP-RGA (Fig. 6C; Supplemental Fig. S5). As in the yeast assay, ATS.2 did not bind to GAI or RGA in plants. This further confirmed the DELLA-ATS interaction in vivo.

ATS directly binds to DELLAs. A, Gene structure of the ATS.1 and ATS.2 splicing variants. The putative domain required for binding is localized in the 5 and 6 exons of ATS.1. B, Yeast two-hybrid assay. DELLAs GAI and RGA fused to Gal4 DNA-BD were tested with ATS.1 and ATS.2 full-length ORFs fused to Gal4 DNA-AD. Diploids were grown in the SD/-Leu-Trp and SD/-Leu-Trp-His-Ade medium. C, BiFC assay. Full-length GAI and RGA in pYFC43 were assayed with ATS.1 and ATS.2 in pYFN43. AKIMβ2 in pYFC43 was used as a negative control (Belda-Palazón et al., 2012). Left, bright field image; right, merged image of chlorophyll and YFP. Scale bar = 100 µm.

If DELLA proteins regulate integument development, it is necessary that they are expressed in these tissues, along with ATS. All four DELLAs, GAI, RGA, RGL1, and RGL2, were expressed in developing ovules (Fig. 7B) but with gene-specific patterns: GAI expression was located mainly in the funiculus but also in integument primordia and in the center of the nucellus; RGA and RGL1 were expressed in the funiculus and integument primordia; and RGL2 was expressed preferentially in the nucellus but also in the integument and funiculus. Therefore, the four DELLAs that were genetically involved in ovule development seemed expressed in the integuments during development, in the same tissues where ATS is expressed (Supplemental Fig. S6A; McAbee et al., 2006) as well as in other tissues within the ovule.

ATS Expression in Integuments during Ovule Development Is Not Regulated by GAs

We also interrogated whether ATS and DELLAs regulate each other at the transcriptional level. First, we tested that there were no differences in the expression of ATS in the wild-type and global ovules (Supplemental Fig. S6A). In both cases, ATS expression was observed in the abaxial cells of the inner integument and in the adaxial cells of the outer integument in developing ovules, as it was previously reported (McAbee et al., 2006). The qPCR analysis revealed that ATS was expressed at similar levels in both inflorescences and seedlings of the global mutant and wild type (Supplemental Fig. S6B), which confirmed in situ hybridization data. On the other hand, the expression of DELLA genes GAI and RGA was not significantly affected in either the seedlings or inflorescences of ats-1 (Supplemental Fig. S6C). We conclude from these results that DELLA proteins would not be involved in the regulation of ATS expression and that ATS would not regulate DELLA gene expression. Therefore, it seems that no reciprocal transcriptional regulation exits between ATS and DELLAs.

Both ATS and DELLA Functions Are Required for Correct Ovule Development

The similar phenotypes of the loss-of-function of ATS and DELLA and the physical interaction showed herein suggested that presence of a protein complex participated by both proteins could be a strict requirement for correct ovule development. Regardless of the interaction capacity of ATS to DELLA, elevated GA levels in ats-1 could be the cause of the described ovule defects; increased GA levels would result in the destabilization of DELLAs and would, hence, promote similar ovule defects to those in the global mutant. In this scenario, a dominant DELLA in an ats-1 background would overcome ovule defects by reestablishing the proper DELLA function. As observed in Figure 8, the gain-of-function of DELLA in the gai-1 mutant did not rescue the ovule and seed phenotypes of ats-1, which supports the notion that both ATS and DELLAs have to be present to promote integument differentiation. While gai-1 showed elongated seeds, the double gai-1 ats-1 had round seeds that were similar in shape and length/width ratio to those from ats-1 (Fig. 8). In addition, the seeds of the double ats-1 gai-1 showed viviparism, as in the single ats-1 and the global mutant (Fig. 8A, inset). In contrast, the overall seed size of gai-1 ats-1 was somewhat larger than those from the single ats-1 (Fig. 8, B and C), which indicated that the gain-of-function of GAI may promote cell elongation in both the major and minor axes of seeds (Vivian-Smith and Koltunow, 1999). The fact that the dominant gai-1 was unable to overcome ovule defects in ats-1 implies that the ATS-DELLA physical protein interaction may be the molecular mechanism by which GAs play a role in integument development.

Gain-of-function of DELLA activity does not rescue the ats-1 mutant defects in seed development. A, Images of 8-dpa fruits from Ler and the ats-1, gai-1, and double ats-1 gai-1 mutants. The white arrow marks a viviparous ats-1 gai-1 seed. Scale bars = 500 µm. B, Seed length and width of gai-1, ats-1, and double ats-1 gai-1. Data are the mean ± sem of three biological replicas, each one from 25 to 30 seeds. Significant differences (Student’s t test analysis) are marked with asterisks, and ats-1 is taken as a reference (*P value < 0.01). The seed length and width ratio is indicated. C, Size distribution of the mature seeds of the ats-1, gai-1, and double ats-1 gai-1 mutants, represented as length (major axis) and width (minor axis). Golden ratio is represented by a line at the 1,618 ratio.

DISCUSSION

In this study, we report the role of GAs in the control of integument development during ovule ontogenesis. Our data indicate that DELLA proteins play an essential role in the correct formation of ovule integument, while constitutive GA signaling has detrimental effects. The similarity of the global and ats-1 mutant phenotypes strongly suggests that both DELLAs and ATS participate in integument growth during late stages of ovule ontogeny via a common molecular mechanism (Fig. 9). ATS and DELLAs interact to coordinate proper ovule growth. This function would require the presence of both proteins in a complex and would explain why the absence of either of them in the global or ats-1 mutants results in similar ovule and seed phenotypes. In line with this, GA biosynthesis repression by ATS may promote the stabilization of DELLAs, which would strengthen the protein complex (Fig. 9). The fact that ats-1 but not global shows fused integuments from early stages would indicate that the role of ATS in this phase of development would not require the activity of DELLAs. Likewise, DELLAs would also have roles in ovule and seed development in other tissues where ATS is not active.

Model for the ATS and DELLA interaction in the control of ovule development. ATS and DELLA directly bind to mediate proper ovule development, probably by regulating the expression of the genes required for ovule development. In addition, ATS inhibits GA biosynthesis, which promotes low GA levels and the stabilization of DELLAs to facilitate the formation of the protein complex.

DELLAs can modulate transcription by binding to TFs in the promoter of target genes (Marín-de la Rosa et al., 2015; Davière and Achard, 2016). It was recently reported that RGA was localized at cis elements that are potential binding motifs of the GARP-G2 TFs that include ATS/KAN4 and KAN1 (Franco-Zorrilla et al., 2014; Marín-de la Rosa et al., 2015). These data strongly suggest that ATS could recruit DELLAs to the promoters of the ATS target genes involved in integument growth through direct protein-protein interaction. In addition, the binding of ATS and DELLA in ovule development resembles that of DELLAs and other GARP genes, the type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) in root growth. In this case, RGA binds to ARRs in the promoters of cytokinin-regulated genes, acting as transcriptional coactivators (Marín-de la Rosa et al., 2015). This molecular mechanism depends on the necessity of the simultaneous presence of DELLAs and ARRs to restrict root meristem growth and to promote photomorphogenesis, similarly to the DELLA-ATS function in ovule integument. RGA selectively induces the expression of ARR1 in the meristem (Moubayidin et al., 2010), which may contribute to facilitate ARR1-DELLA complex formation to arrest meristem growth. In contrast, the expression of DELLAs is not regulated by ATS or vice versa. In the case of the integument, repression of GA biosynthesis by ATS would stabilize DELLA proteins and favors the ATS-DELLA complex formation.

ATS also interacts with ETT/ARF3 to define the boundary between the integument primordia in the ovule (Kelley et al., 2012). The loss-of-function ATS or ETT resulted in common defects in ovule integument development and seed shape, probably by altering auxin distribution in ovule primordia. Interestingly, DELLAs can also bind to ETT (M. Blazquez and D. Alabadi, unpublished data), which points out to a multicomplex protein DELLA-ATS-ETT that could coordinate ovule growth and development.

Early seed germination within siliques was observed in the global and ats-1 mutants. It is well known that GAs and ABA play antagonistic roles in seed dormancy/germination by means of a dynamic balance between the synthesis and catabolism of these two hormones (Gutierrez et al., 2007; Holdsworth et al., 2008; Weitbrecht et al., 2011). On the other hand, reduced dormancy could be a direct consequence of the diminished mechanical resistance in the testa caused for the loss-of-function of the DELLA-ATS complex. It is difficult to discriminate whether viviparism is merely the consequence of testa defects or the result of the constitutive GA signaling (global) or elevated GA levels (ats-1). However, viviparism was still observed in the double ats-1 gai-1, which has blocked the GAI-dependent GA signaling, suggesting that defects in the testa of ats-1 are potentially responsible for the reduced dormancy. ats-1 also showed early germination and advance flowering, similarly to the global mutant. Whether these phenotypes are the effect of the elevated GA levels or due to a specific role of ATS-DELLA, as it was observed in ovule development, needs further study.

In conclusion, we have shown that the correct control of GA/DELLA levels is necessary to achieve proper integument development and to generate a normal testa in mature seeds. Constitutive GA signaling by diminished DELLA function results in changes in the ovule/seed shape due to alterations in integuments/testa, which have consequences in both fertility and seed dormancy.

MATERIALS AND METHODS

Seeds were surface-sterilized in ethanol, plated onto Murashige and Skoog medium (Murashige and Skoog, 1962), incubated at 4°C for 4 d in complete darkness, and transferred to a growth chamber at 22°C in long-day photoperiod (16/8 h) for 10 d. Seedlings were transferred to soil (a mix of peat moss, vermiculite, and perlite, 2:1:1) and grown in a growth chamber at 22°C in long-day photoperiod.

Fertility was assayed in hand-emasculated flowers 1 d before anthesis and hand pollinated with wild-type pollen at anthesis. Fruits were collected at maturity (14 d postanthesis [dpa]), seed number was counted, and silique length was measured with a digital caliper. The ratio (seed number versus silique length) was determined. Parthenocarpy was assayed in hand-emasculated flowers, which were not pollinated.

Germination was assayed in 2- to 3-week-old seeds, plated in Murashige and Skoog medium, and stratified for 3 d at 4°C in the darkness. Germination was scored 3 d after incubation at 22°C as radicle emergence through the testa. Flowering was assayed under long-day (16/8 h) and short-day (8/16 h) conditions. After stratifying for 3 d at 4°C in the darkness, seeds were germinated directly in soil and grown at 22°C. The total number of leaves before flowering was scored.

Gene Expression Analysis by qPCR

Three-day-old seedlings and inflorescences were used. Total RNA was extracted with the RNeasy Plant Mini Kit (Qiagen) and genomic DNA was eliminated with DNase I (Qiagen). cDNA was synthesized using the SuperScript First-Strand Synthesis System for RT-PCR (Invitrogen), and qPCR was carried out with the SYBR Green PCR Master Mix (Applied Biosystems) with an ABI PRISM 7000 Sequence Detection System (Applied Biosystems), as previously described (Dorcey et al., 2009). Expression levels were calculated according to the expression of the constitutive EIF4A1 (At3g13929) and data were normalized by the ΔΔCt method. The primers for the splicing version ATS.1 (forward 5′-CAACTCATCACACTAAGGACAATGAAG-3′, which spanned the intron between exon 5 and 6, and reverse oligo 5′-CCAAATTGAGATGAATGTTGGTATCC-3′) were tested for efficiency. The primers for GAI, RGA, and GA biosynthesis genes were previously described (Dorcey et al., 2009; Gallego-Giraldo et al., 2014).

Scanning Electron Microscopy

Seeds were mounted on scanning electron microscopy stubs attached to the specimen holder of a CT-1000C cryo-transfer system (Oxford Instruments) and frozen in liquid N2. The frozen samples were transferred to the cryo-stage of a JEOL JSM-5410 scanning electron microscope, sublimated at −85°C, and sputter coated with a film of gold. Finally, samples were observed at incident electron energy of 10 kV.

Histological Procedures

The pGA3ox1:GA3ox1-GUS (Mitchum et al., 2006) and pDELLA:GUS lines (Gallego-Giraldo et al., 2014) were used for the GUS assay and the histological procedures basically as previously described (Carbonell-Bejerano et al., 2010). The K3Fe(CN)6 and K4Fe(CN)6 concentrations were adjusted for each line to obtain optimal signals (5 mm for pGAI:GUS and pRGA:GUS, 2 mm for pRGL1:GUS, 4 mm for rgl2-5, and 50 µm for pGA3ox1:GA3ox1-GUS). After GUS staining, samples were either cleared with chloral hydrate or stained following a modified pseudo-Schiff propidium iodide technique (Truernit et al., 2008). Images were captured with a microscope Eclipse E600 (Nikon) equipped with Nomarski interference optics or with a ZEISS LSM 780 confocal microscope. GUS staining was visualized in the confocal with a MBS T80/R20 dichroic (561 nm and 545–570 nm excitation and reflection, respectively). PI staining was excited at 561 nm and detected at 580 to 660 nm.

For thin sectioning in Figure 3, D and E, fruits were fixed overnight in 4% (w/v) p-formaldehyde in 0.1 m sodium phosphate, pH 7.2, with 0.05% (v/v) of Tween 20 at 4°C, and dehydrated in ethanol. Samples were then infiltrated in Technovit 7100 resin, sectioned in a Reichert Jung Ultracut E microtome at 3 μm, and stained in 0.02% Toluidine blue as described by Gómez et al. (2004).

In Situ RNA Hybridization

Inflorescences were embedded, sectioned, and hybridized as described elsewhere (Weigel and Glazebrook, 2002; Gomez et al., 2011). The partial clone for ATS.1, corresponding to the complete codifying sequence, was cloned in the pGEM-T easy vector (Promega). Sense and antisense probes were synthesized using the SP6 and T7 RNA polymerases, respectively.

Yeast Two-Hybrid

Full-length open reading frames (ORFs) were first cloned in pGEM-T (ATS.1 and ATS.2) or pBSK (KAN1), and transferred into pGADT7 (Clontech) by EcoRI-ClaI digestion-ligation. The M5-truncated versions of cDNA for GAI and RGA (de Lucas et al., 2008) in pGBKT7 (Clontech) were obtained from S. Prat (CNB, Spain). Plasmids pGADT7 and pGBKT7 were introduced into the Y187 and the Y2HGold yeast strain, respectively. Both strains containing the corresponding plasmids were mated overnight at 28°C, and diploids were selected in the synthetic dextrose media supplemented with His and Ade. The interaction was assayed on minimal media plates without either Leu/Trp or Leu/Trp/Ade/His in 10-fold serial dilutions for 5 d at 28°C. Empty vectors were used as negative controls.

The BiFC assay was carried out basically as described by Belda-Palazón et al. (2012). The full-length ORFs of ATS.1/ATS.2 and GAI/RGA were cloned in pCR8 and transferred by Gateway into the YFN43 and the YFC43 plasmid, respectively. As a nonspecific control, cDNAs for AKINβ2 in pYFC43 and AKIN10 in pYFN43 were used (Ferrando et al., 2001). Fluorescence was observed 2 to 3 d after infiltration at 488 nm under a Zeiss LSM 780 confocal microscope.

Supplemental Data

The following supplemental materials are available.

Supplemental Figure S1. Parthenocarpy and seed defects are characteristic of the global and quadruple but not of any of the triple della null mutants.

Footnotes

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Miguel A. Perez-Amador (mpereza{at}ibmcp.upv.es).

M.D.G. performed most of the experiments; D.V. carried out protein-protein binding assays; R.S. analyzed DELLA mutants; M.A.P.-A. performed mutant seed analysis; M.D.G. and M.A.P.-A. conceived the project, analyzed and interpreted data, and wrote the article with contributions by all the authors.

↵1 This work was supported by grants BIO2011-26302 and BIO2014-55946 from the Spanish Ministry of Science and Innovation and the Spanish Ministry of Economy and Competitiveness, respectively, and ACOMP/2013/048 and ACOMP/2014/106 from the Generalitat Valenciana to M.A.P.-A. R.S. received a PhD fellowship from the Spanish Ministry of Science and Innovation.

(2010) Modeling the Arabidopsis seed shape by a cardioid: efficacy of the adjustment with a scale change with factor equal to the Golden Ratio and analysis of seed shape in ethylene mutants. J Plant Physiol167: 408–410